1. Technical Field
The embodiments discussed herein generally relate to a spin torque oscillator capable of stable high frequency oscillation, a magnetic recording head adapted for high magnetic recording density and having a spin torque oscillator for microwave assisted recording, and a magnetic recording apparatus.
2. Related Art
In recent years, spin electronics elements, such as TMR (Tunneling Magneto Resistance) heads used as reproduction heads in HDDs (Hard Disk Drives), or spin injection MRAMs (Magnetic Random Access Memories), have made a significant contribution to the progress of information technology. In the course of development of such spin electronics elements, various discoveries have been made, such as the spin torque oscillator that generates microwaves by causing a ferromagnetic material to oscillate by using spin torque, and the spin torque diode effect by which high frequency current is rectified. Thus, an increasing range of possible applications is opening up for high frequency generation, detection, modulation, and amplification, for example. Particularly, the application of the spin torque oscillator in HDDs is gaining much attention as a way of achieving future high recording density. In the following, the related art will be described in greater detail with regard to problems facing the increase in recording density for HDDs and methods for achieving high recording density by solving the problems.
The recording density of HDDs has been increased over the years and the bit size of the recording medium has become increasingly smaller. However, as the bit size becomes smaller, the loss of a recording state due to thermal fluctuation has become an issue of concern. In order to solve the problem and ensure stable maintenance of recording bits at the high density recording of the future, it is necessary to use a recording medium with large coercivity, i.e., large magnetic anisotropy. And in order to record a recording medium with large coercivity, a strong recording magnetic field is required. In practice, however, there is an upper limit to the recording magnetic field intensity due to limitations to the decrease in size of the recording head and the available magnetic materials. For these reasons, the coercivity of the recording medium is restricted by the magnitude of the recording magnetic field that the recording head can generate. In order to address the contradicting demands for high thermal stability of the medium and the coercivity permitting easy recording, a recording technique has been devised whereby the coercivity of the recording medium is effectively lowered only during recording by using various auxiliary measures. A representative example is the thermally assisted recording, by which recording is performed by using a magnetic head and a heating unit, such as a laser, in combination.
According to another idea, recording is performed by using a high-frequency magnetic field in combination with a recording magnetic field from the recording head so that the coercivity of the recording medium can be locally decreased. For example, JP Patent Publication (Kokai) No. 6-243527 A (1994) discusses a technology such that the magnetic recording medium is heated by Joule heating or magnetic resonance by using a high-frequency magnetic field so that information can be recorded by locally decreasing the coercivity of the medium. The recording technique utilizes the magnetic resonance of the high-frequency magnetic field and the magnetic field from the magnetic head. Because the technique (hereafter referred to as “microwave assisted recording”) utilizes magnetic resonance, a strong high-frequency magnetic field in proportion to the anisotropic magnetic field of the medium needs to be applied in order to obtain the effect of decreasing the switching magnetic field.
In recent years, the principle of generating a high-frequency magnetic field by using spin torque has been proposed, such as in the form of the spin torque oscillator, making the possibility of microwave assisted recording more realistic. For example, X. Zhu and J. G. Zhu discuss the result of a calculation regarding a spin torque oscillator that oscillates without an external bias magnetic field in “Bias-Field-Free Microwave Oscillator Driven by Perpendicularly Polarized Spin Current”, IEEE TRANSACTIONS ON MAGNETICS, P 2670, VOL. 42, NO. 10 (2006). J. G. Zhu and X. Zhu also discuss a technology for recording information in a magnetic recording medium with a large magnetic anisotropy in “Microwave assisted Magnetic Recording”, The Magnetic Recording Conference (TMRC) 2007 Paper B6 (2007). According to this technology, a microwave (high-frequency magnetic field) is generated by disposing a magnetization high-speed rotating body or a field generation layer (FGL), in which magnetization is caused to rotate at high speed by spin torque, in the vicinity of a magnetic recording medium adjacent to the main magnetic pole of a perpendicular magnetic recording head. J. Zhu and Y. Wang present a spin torque oscillator such that the rotating direction of the FGL is controlled by utilizing the magnetic field of the main magnetic pole in proximity to the FGL in “Microwave assisted Magnetic Recording with Circular AC Field Generated by Spin Torque Transfer”, MMM Conference 2008 Paper GA-02(2008), whereby microwave assisted magnetization reversal of the medium can supposedly be efficiently achieved.
Other related technologies are discussed in JP Patent Publication (Kokai) No. 2009-49101 A and by Mizukami et al., Phy. Rev., B 66, 104413 (2002).